drm/ast: Use drm_gem_vram_{vmap,vunmap}() to map cursor source BO

The VRAM helper's vmap interfaces provide pinning and mapping of BO
memory. This patch replaces the respective code in ast cursor handling.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911120352.20084-3-tzimmermann@suse.de